It is desirable in many circumstances to deliver a fluid to a pressurized target location at a fixed flow rate. This may be achieved by communicating fluid from a pressurised source to the target location via a flow restriction or orifice. In such a case the flow rate is, generally, a function of a flow coefficient of the restriction and the pressure differential between the fluid source and target location. The flow rate through a restriction or orifice may be formulaically expressed by:
  Q  =      K    ⁢                            Δ          ⁢                                          ⁢          P                Sg            Where:Q=flow rateK=flow coefficient of the restrictionΔP=the differential pressure over the flow restrictionSg=specific gravity of the fluid.
In many applications the specific gravity of the fluid will be constant. Accordingly, it will be recognised that if the flow restriction and the pressure differential are fixed, then a steady state flow rate from the source to the target may be achieved. However, in some circumstances one or both of the source and target location pressures may vary or fluctuate, thus causing a variation in the pressure differential. This will result in fluctuations in the flow rate which may be undesirable.
To address the issue of fluctuating flowrates, it is possible to employ a variable flow restriction which is varied in accordance with variations in the differential pressure. However, such an arrangement typically requires the use of complex control equipment for monitoring the various pressures and operating some form of actuator to vary the flow restriction accordingly.
An alternative approach is to ensure that a constant pressure differential is applied across a fixed flow restriction, irrespective of pressure fluctuations at the source and target. An apparatus for use in such a known approach, which is similar to that disclosed in GB 2 238 848, will now be described with reference to FIGS. 1 and 2, wherein FIG. 2 provides an enlarged view of a portion of the apparatus of FIG. 1.
The apparatus, generally identified by reference numeral 210, includes a piston member 212 mounted to stroke within a bore 214 formed within a valve housing 216, wherein a seal 218 is provided between the piston member 212 and the bore wall. The piston member 212 and housing 216 collectively define a lower chamber 220 and an upper chamber 222, isolated from each other by the piston seal 218. The housing defines a fluid inlet 224 which receives fluid from a fluid source 226 and communicates with the lower chamber 220, such that the pressure of fluid within the lower chamber 220 acts against the piston 212, on the area defined by the seal 218, establishing an upward force. As will be discussed in further detail below, the pressure of the fluid is conditioned within the lower chamber upon movement of the piston 212.
The fluid inlet 224 comprises an inlet valve 228 having a spring mounted ball 230 which cooperates with a ball seat 232 to control the inflow of fluid into the lower chamber 220. A pin 234 extends from a lower side of the piston 212 and in use functions to displace the ball 230 relative to its seat 232, upon movement of the piston 212, to vary the flow into the lower chamber 220.
The housing 216 further defines a fluid outlet 236 for the fluid to be delivered from the lower chamber 220 to an inlet 238 of an external fixed flow restrictor 240, wherein fluid from an outlet 242 of the flow restrictor 240 is delivered to a target location 244. The flow restrictor 240 is arranged to deliver fluid to the target location 244 at a desired flow rate.
The housing 216 also defines a further inlet 246 which communicates fluid, or fluid pressure, from the downstream side of the flow restrictor (which will be at the target location pressure) into the upper chamber 222, wherein the fluid pressure will act against the piston 212, on the area defined by the seal 218, establishing a downward force.
The apparatus 210 further comprises a bias spring 248 located within the upper chamber 222 and arranged to establish a downward force on the piston. An adjustor mechanism 250 is provided to vary the bias force of the spring 248.
In use, prior to any flow the bias spring 248 will force the piston 212 downward such that the ball 230 of the inlet valve 228 is lifted from its seat 232 by engagement with the pin 234. On start-up fluid from the fluid source 226 will flow through the inlet valve 228 and into the lower chamber 220 to provide an upward force on the piston 212, which will be opposed by the force of the spring and also the force established by the target location pressure acting in the upper chamber 222. During a dynamic period the piston 212 will be displaced, thus displacing the ball 230 of the valve 228 via the piston pin 234 to regulate the inlet fluid until an equilibrium condition is established. In such an equilibrium condition the forces acting on the piston 212 will be balanced. As the fluid pressure above and below the piston 212 act on an equal seal area, the fluid within the lower chamber will thus be a fixed value above the target location pressure by an amount determined by the force of the spring. Any variations in the pressure at either or both of the source 226 and target location 244 will be accommodated by adjustment of the piston 212 and inlet valve 228 to continuously maintain equilibrium. As such, a constant pressure differential will be applied across the restrictor 240 permitting a constant flow rate to be achieved.
As noted above, the inlet valve 228 is opened by downward movement of the piston 212. However, for the piston 212 to lift the ball 230 from its seat 232 it must overcome the inlet pressure which will be acting on the ball 230 over the area of the seat 232. The effect of the inlet pressure will therefore apply a force against the piston, which may vary as the piston pin 234 engages and disengages the ball 230 when in use. The present inventor has identified this as an adverse issue in that the effect of this varying force established by the inlet pressure acting against the seat 232 will result in variations in the fluid pressure within the lower chamber, and thus adverse variations in the pressure differential across the flow restrictor 240. Such variations in the pressure differential will result in variations in the flow rate.
The effect of the inlet pressure acting against the valve seat may be addressed by forming the apparatus 210 to ensure a large ratio between the area of the piston 212 and the area of the valve seat 232, to minimise the pressure variation effect. However, this establishes the requirement to use a relatively large piston which necessitates a correspondingly large housing. In typical high pressure applications, such as may be experienced in the oil and gas industry, a typical housing size may be far in excess of 75 mm (3 inches). This therefore renders the known apparatus 210 to be generally unsuitable for downhole applications, where target housing sizes may be in the region of 25 mm (1 inch).
It may also be desirable to deliver a fluid at a fixed flow rate to multiple target locations at different pressures. This may be the case in the oil and gas industry, for example where an injection fluid must be injected into different formation zones. One known arrangement for such multiple zone injection is to utilise multiple injection devices which are fed by multiple feed lines. However, this requires the availability of space to accommodate the multiple feed lines, which may be undesirable, and perhaps not achievable in typical downhole environments. It is therefore preferred in some circumstances to utilise a common feed line which delivers fluid to multiple injection devices. However, even where such common feed systems are desired, they may not practically be implemented in view of the typical sizes of injection device, as noted above.